Reconfigurable optical add/drop multiplexers (ROADMs) are increasingly being used in dense-wavelength-division-multiplexed (DWDM) networks to support Layer 1 services, such as private lines. Software-controlled ROADMs manage data traveling over high-capacity fiber optic lines and can automatically detect and adjust bandwidth, move traffic to different lanes, turn off wavelengths for a variety of different reasons, and so forth.
Generally, each ROADM is connected to one or more other ROADMs by one or more optical fiber pairs. A given ROADM will transmit an optical signal on one fiber in a pair and receive a return signal on the other fiber in the pair; thus, each optical fiber transmits in a single direction. A Layer 1 service, or a wavelength, can then be set up between two transponders, where each transponder is connected to a nearby ROADM. The wavelength may then be routed through the ROADM network.